Newer built Uberti 1858s have the text "cal .44 black powder only - A.Uberti Italy" underneath the loading lever, and there's an Uberti logo beside the serial number stamped on the metal below the handle.
Also mechanically speaking, the internals are blued, the muzzle is crowned and front sight is dovetailed, also hammer is checkered in two directions rather than one.
With older guns, don't know. I know Piettas have generally stuff stamped on the side of the barrel.